1. Field of the Invention
This invention relates generally to dielectric resonator oscillators for generating microwave energy, and relates more particularly to a dielectric resonator oscillator that generates microwave energy at any of several fixed frequencies.
2. Description of the Relevant Art
Microwave frequency oscillators are key elements of many communication systems and radars. Fixed frequency microwave oscillators are often implemented in hybrid circuits using transistors (either field effect or bipolar) and dielectric resonators passively coupled to microstrip lines. Two such dielectric resonator oscillators, known in the prior art, are shown in FIGS. 1 and 2.
The oscillator of FIG. 1 includes a dielectric resonator 10 as a series feedback element coupled to the gate terminal of a field effect transistor 12 via a microstrip line 14. The source terminal of the transistor is coupled to ground via a capacitor 16 and via an inductor 18 and resistor 20 wired in series to ground and in parallel to capacitor 16. The drain terminal of the transistor is coupled to the load 22 via a microstrip line 24.
The gate terminal of the transistor 12 and one end of the microstrip line 14 are grounded via inductor 26, while the other end of the microstrip line 14 is grounded via resistor 28. In operation, a positive direct current voltage is applied to the drain terminal of the transistor, and the resistor 20 determines the biasing condition (i.e., drain current) of the transistor. The transistor 12 oscillates at a frequency equal to the resonant frequency of the dielectric resonator 10.
The oscillator of FIG. 2 includes a dielectric resonator 30 as a stabilization element coupled to the drain terminal of a field effect transistor 32. The transistor 32 of FIG. 2 is biased in the same way as the transistor 12 of FIG. 1. A stub length 34 of microstrip line is connected to the gate terminal of the transistor 32. Without the dielectric resonator 30, the oscillator would be free running, but with the resonator, the oscillator stabilizes at the resonant frequency of the resonator.
While such single frequency oscillators are useful, there is also a need for microwave oscillators that can selectively generate any of several fixed frequencies. The design requirements of such an oscillator include (1) output frequency selection from several available frequencies, (2) stable output frequencies, (3) fast switching between frequencies, and (4) no spurious signals.
One prior art approach is to join together several dielectric resonator oscillators, as shown in FIG. 3. Three separate dielectric resonator oscillators, each with a different operational frequency, are selectively connected to a load 36 via a switch 38, which is shown schematically as a single-pole, triple-throw switch. The switch 38 may be implemented using PIN type diodes. Often an additional transistor is used between each dielectric resonator oscillator and the switch 38 for output signal amplification and to isolate the oscillator from the load.
In some implementations of such a multiple frequency device, all of the dielectric resonator oscillators continuously operate in order to provide stable operation and to allow fast switching from one frequency to another. In theory, only the signal generated by the selected dielectric resonator oscillator is supplied to the output terminal. In actuality, however, signals from the non-selected dielectric resonator oscillators leak through the switch to create unwanted spurious signals in the output signal. Extremely high isolation switches are required to reduce the leakage through the switch. The isolation values required are generally difficult to meet as a practical matter even with complex and expensive multi-throw switches, particularly within the X and Ku bands. The presence of spurious signals can be a very severe problem in certain electronic warfare systems, wherein a spurious signal may be erroneously interpreted as a threat signal.
In other implementations of the prior art multiple frequency device, the dielectric resonator oscillators are switched on only when needed to generate the output signal. While this approach eliminates the spurious signal problem, it greatly increases the switching time because one oscillator must be switched on and another oscillator must be switched off each time the output frequency is changed. In addition, frequency of the output signal may wander somewhat before the selected dielectric resonator oscillator stabilizes. Thus, this implementation suffers from the drawbacks of increased switching time and decreased stability.